1. Field of the Invention
The invention relates to a toner, a two-component developer, a developing device, and an image forming apparatus.
2. Description of the Related Art
In recent years, image forming apparatuses employing the electrophotographic process have rapidly been widespread, such as electrostatic copiers or laser beam printers, among which the image forming apparatuses capable of printing in full color are notable.
The image forming apparatus employing the electrophotographic process indicates an apparatus for forming an image on a recording medium such as paper or a sheet through a charging step, an exposure step, a developing step, a transferring step, and a fixing step. In the charging step, a surface of a photoreceptor is evenly charged. In the exposure step, the charge photoreceptor is exposed to light so that an electrostatic latent image is formed on the surface of the photoreceptor. In the developing step, a developer adheres to the electrostatic latent image formed on the surface of the photoreceptor to thereby form a visualized image. In the transferring step, the visualized image formed on the surface of the photoreceptor is transferred to the recording medium. In the fixing step, the visualized image transferred to the recording medium is fixed by heat, pressure, and the like. Through the steps as above, a desired image is formed on the recording medium in the image forming apparatus employing the electrophotographic process.
In a typical image forming apparatus employing the electrophotographic process, full color printing is done with a developer formed of the toners of three colors, i.e., yellow, magenta, and cyan which are three primary colors, or the toners of four colors including black in addition to those three colors. That is to say, the toners of respective colors are all treated in the charging step, the exposure step, the developing step, and the transferring step, to thereby form on the recording medium the visualized image which is composed of the toners of plural colors and which is then fixed onto the recording medium in the fixing step by melting, blending and mixing colors of the toners, thus resulting in a full color image.
With the aim of enhancing the low-temperature fixing property which is one of problems in the fixing step, there have been proposed so far the techniques of using the toner in which a binder resin contains an amorphous resin having a low glass transition temperature, and the toner which contains wax having a low melting temperature. These techniques do not, however, provide sufficient low-temperature fixing property and besides, they have a problem that the preservation stability of the toner deteriorates by the addition of a large amount of the amorphous resin or the wax having a low melting temperature.
Other than the above techniques, the technique of using crystalline polyester having a better low-temperature fixing property as a binder resin has been proposed. This technique has, however, a problem that by using the crystalline polyester alone, the preservation stability, anti-offset property, and the like property deteriorate and thereby narrows the temperature range where the toner can be fixed. And in the case where the crystalline polyester and the amorphous resin are used together, it is difficult for the crystalline polyester to be sufficiently dispersed in the binder resin, and a problem thus arises that favorable charge uniformity is hard to obtain. To deal with the problem, Japanese Unexamined Patent Publication JP-A 2002-287426 discloses an electrophotographic toner which contains the binder resin containing the crystalline polyester and the amorphous resin wherein the crystalline polyester is contained in the binder resin in an amount of from 1% by weight to 40% by weight and wherein 90% or more of a dispersed domain of the crystalline polyester has a diameter of from 0.1 μm to 2 μm, and the dispersibility of the crystalline polyester is thereby adjusted to a proper level so that favorable low-temperature fixing property and charge uniformity are provided.
By the way, various studies have been conducted on an external additive for the purpose of improving the chargeability and flowability of the toner which is one of problems in the developing step and the transferring step. For example, the technique of using conductive fine particles of titanium oxide or the like substance as an external additive for the toner has been proposed in order to stably control a charge amount by moving charges smoothly between toner particles and between the toner and a carrier.
The toner disclosed in Japanese Unexamined Patent Publication JP-A 4-452 (1992) contains titanium oxide treated with a fatty acid metal salt and thereby provides favorable charge stability with a quicker initial rise of charging a newly supplied toner. Further, the dry developer for electrostatic charge image development disclosed in Japanese Unexamined Patent Publication JP-A 2001-83731 contains toner particles containing a binder resin and a colorant, and as a fluidizer (an external additive), amorphous silicon-aluminum co-oxidized fine particles in which Al2O3 and SiO2 are mixed in predetermined proportion, to thereby prevent the toner particles from being excessively charged so that the charge amount of the toner is maintained at an optimal level for a long period of time.
Moreover, in recent years, numerous efforts have been made in various technical fields from the aspect of environmental conservation. Today, petroleum is used as a raw material of many products, and most of the binder resin which accounts for 70% of constituents of a toner, is also manufactured by using an oil resource as a raw material. To reduce this oil resource use is very important not only from the viewpoint of solving the problem that the oil resource is running out, but also from the viewpoint of prevention of global warming because the amount of carbon dioxide generated in manufacturing or incinerating the oil resource is reduced.
This is why a lot of attention is now given to the use of a plant-derived resource called biomass. Because the carbon dioxide generated in burning the biomass originates from the carbon dioxide which used to exist in the air and has been taken in a plant by photosynthesis, the whole balance of input and output amounts of carbon dioxide in the air is zero, i.e., the total amount of carbon dioxide in the air does not change. Accordingly, if such a plant-derived resource as biomass can be used as a raw material to synthesize a binder resin for toner, the amount of carbon dioxide in the air can be fixed and it would be thus possible to solve the problem of depletion of oil resources and the problem of global warming simultaneously.
For example, the electrophotographic toner disclosed in Japanese Unexamined Patent Publication JP-A 2001-166537 contains as a binder resin a resin which contains a biomass-derived polylactic acid-based biodegradable resin and a terpene phenol copolymer, and exhibits an enhanced low-temperature toner fixing property.
In JP-A 2002-287426, no consideration is given to control of an external additive on a charge amount of the toner containing crystalline polyester with low chargeability, and long-term charge stability of the toner may be therefore decreased.
In JP-A 4-452, titanium oxide which deteriorates transparency of the toner is used and the balance between chargeability and color reproducibility of the toner is not considered, and the color reproducibility may therefore deteriorate in the case of using a color toner which contains crystalline polyester having lower transparency.
In the technique disclosed in JP-A 2001-83731, the toner containing crystalline polyester is not taken into consideration and therefore, in the case of using the toner containing the crystalline polyester with low chargeability, the long-term charge stability may be decreased even with the use of amorphous silicon-aluminum co-oxidized fine particles adjusted to the disclosed mix proportion.
In JP-A 2001-166537, the crystalline polyester, i.e., polylactic acid resin is used, but the dispersibility of the crystalline polyester is not considered, and it therefore may not be possible to obtain a favorable fixing property.